A typical domestic septic tank is a watertight container that receives wastewater from toilets, sinks, and household appliances, provides a primary level of treatment of the wastewater, and then releases the primary treated wastewater to a drainfield or further treatment equipment. A simple septic tank may have rectangular or cylindrical shape. The wastewater is received from the source through an inlet and travels the length of the tank to an outlet. While in the tank the wastewater separates into three layers. As the wastewater flows through the tank from the inlet to the outlet, solids tend to fall to the bottom of the tank and form a sludge layer. Grease, light floating particles, and gas-buoyed sludge rise toward the surface of the wastewater to form a scum layer. A middle layer between the sludge and the scum is referred to as the clear layer. Sometimes upper and lower baffles are arranged perpendicular to the flow to create chambers that retain the sludge and scum. At the downstream end of the tank, primary treated wastewater from the clear layer is discharged from the outlet while the sludge and scum are retained in the tank and are periodically pumped out.
The flow of wastewater is often delivered to the tank in surges. In the tank the flow of wastewater slows down, allowing the solids to drop out and the scum layer to form. Generally, the longer the wastewater resides in the tank, the more solids and scum migrate out of the clear layer. While a large tank increases the residual time, large tanks are more expensive to construct, transport, and install. Maximizing the distance that wastewater travels between the inlet and the outlet also encourages the separation of wastewater into the three layers.
A meander tank is one way to increase the length of flow of wastewater from the inlet 18 to the outlet while not increasing the size of the tank. A meander tank uses one or more baffles generally parallel to the flow to create zones or chambers and increase length of flow between inlet and outlet. Two examples of rectangular meander tanks are disclosed in CEE 484 Decentralized and Onsite Wastewater Management and Reuse, Septic Tanks, Apr. 4, 2007, H. David Stenzel. One such meander tank uses a baffle parallel to the walls of the tank to create two longitudinal chambers causing the wastewater to flow substantially twice the length of the tank from the inlet to outlet. Another meander tank uses two baffles to create three chambers requiring the wastewater to travel approximately three times the length of the tank to reach the outlet.
A disadvantage of the rectangular meander tanks shown in CEE 484 is that the corners create dead spaces where there is little flow and thus portions of the tank do not contribute to length of flow of wastewater as it moves through the tank. Put another way, the full volume of the tank could be used to contribute to the length of flow if there were no dead spaces.